QUANTITATIVE SPECTROSCOPICJ-BAND STUDY OF RED SUPERGIANTS IN PERSEUS OB-1

Abstract

We demonstrate how the metallicities of red supergiant (RSG) stars can be measured from quantitative spectroscopy down to resolutions of ≈ 3000 in the J-band. We have obtained high resolution spectra on a sample of the RSG population of h and χ Persei, a double cluster in the solar neighborhood. We show that careful application of the marcs model atmospheres returns measurements of Z consistent with solar metallicity. Using two grids of synthetic spectra−one in pure LTE and one with NLTE calculations for the most important diagnostic lines−we measure Z = +0.04 ± 0.10 (LTE) and Z = −0.04 ± 0.08 (NLTE) for the sample of eleven RSGs in the cluster. We degrade the spectral resolution of our observations and find that those values remain consistent down to resolutions of less than λ/δλ of 3000. Using measurements of effective temperatures we compare our results with stellar evolution theory and find good agreement. We construct a synthetic cluster spectrum and find that analyzing this composite spectrum with single-star RSG models returns an accurate metallicity. We conclude that the RSGs make ideal targets in the near infrared for measuring the metallicities of star forming galaxies out to 7-10 Mpc and up to ten times farther by observing the integrated light of unresolved super star clusters.

Red Supergiants as Cosmic Abundance Probes: The Magellanic Clouds

“…The distribution of metallicities is peaked at ∼−0.6 dex but with a high metallicity tail (see Figure 7). We also find that the average value of ξ is slightly lower in the SMC than for the LMC and for the Per OB1 stars studied in Gazak et al (2014) ( 2.8 0.3 SMC ξ = ± km s −1 , as opposed to 3.3 ± 0.5 and 3.6 ± 0.5 km s −1 for the LMC and Per OB1 respectively; see Figure 8). Though there does seem to be a weak trend of decreasing ξ at lower metallicities, this is not borne out by a study of RSGs in NGC 6822, a galaxy found to have similar metallicity to the SMC (Patrick et al 2015).…”

“…Allowing for a contribution from convective pressure which would reduce the effective gravity by up to 0.3 dex (Chiavassa et al 2011;Gazak et al 2014), we can identify the extreme possible values of g log . When taking the 2 χ -weighted mean of each of the parameters within this range, or simply just their values at the 2 χ minimum, we find very similar results to those by the PCA analysis (see above).…”

“…In Gazak et al (2014), we performed a comprehensive test of this technique within a solar metallicity environment, analyzing a sample of stars in the Milky Way open cluster Per OB1. We found an average metallicity Z 0.04 0.08 = − ± , consistent with the results from BSGs within the same cluster and in the solar neighborhood (Firnstein & Przybilla 2012;Nieva & Przybilla 2012).…”

The evolution of red supergiant mass-loss rates

“…In contrast Gazak et al (2014) found a narrower T eff spread among the stars in χ Per, 3720K -4040K. In this work, we have opted for a fiducial SED of 3900K for the analysis of this cluster, with the errors on L bol found by rerunning the analysis with SEDs of temperatures ± 300K fully encompassing the observed range of both Gazak et al (2014) and Levesque et al (2005). For the NGC 7419 RSGs, the spectral types range from M0 to M7.5 (Marco & Negueruela 2013) corresponding to a temperature range of 3400 -3800K.…”

Red Supergiant Stars as Cosmic Abundance Probes: Kmos Observations in NGC 6822

“…To realise this goal, Davies et al (2010) outlined a technique to derive metallicities of RSGs at moderate spectral resolving power (R ∼ 3000). This technique has recently been refined using observations of RSGs in the Magellanic Clouds (Davies et al 2015) and Perseus OB-1 (Gazak et al 2014b). Using absorption lines in the J-band from iron, silicon and titanium, one can estimate metallicity ([Z] = log Z/Z e ) and other stellar parameters (effective temperature, surface gravity and microturbulence) by fitting synthetic spectra to the observations.…”